In fairly small chip burning plants of a size class typically below 2 MW, the use of boilers planned for burning dry chips is most preferable, as far as purchase price is concerned. Such plants are poorly adapted to variations in the moisture of the fuel, i.e. they are not capable of burning for instance, unseasoned chips, bark or sawdust. When boilers are planned in such a way that also unseasoned fuel can be burned in them, the price of the plant easily rises higher. Primary price increasing factors are additional masonry for the combustion chamber, an enlarged convection part and preheaters of combustion air. A plant planned for burning moist fuel is also poorly suited for dry fuel, because dry fuel raises the temperature of the combustion chamber high and the plant will be subjected to resistance problems, for instance. If necessary, dry fuel must even be wetted. A plant planned for burning moist fuel is also poorly suited for small power level requirements because of difficult power control, particularly when dry fuel is used.
Finnish publication 780 822 discloses a fuel drying apparatus of a boiler. The drying apparatus comprises a fuel tank, from which the fuel is transported to a combustion chamber of the boiler. Combustion gases from the boiler are led into a closed space below the fuel tank, whereby the combustion gases heat the fuel. By this arrangement, moist fuel can be dried before it is led into the boiler, but when already dry fuel is fed into the fuel tank, the combustion gases dry it further completely unnecessarily, which causes a fire risk in the fuel tank, for instance.
Finnish publication 60 435 also discloses a fuel drying apparatus. The drying apparatus comprises a vertically longitudinal flat chamber heated by combustion gases, in which chamber there are horizontal screw conveyors on each other and under the screw conveyors chutes, in which the fuel passes from one screw conveyor to another back and forth and downwards. The combustion gases are directed to flow upwards in the chamber, whereby they dry fuel. The arrangement is inconvenient and complicated and the drying procedure cannot be controlled in any way, which means that when, for instance, dry fuel is fed it is dried unnecessarily and there is an obvious fire risk in the drying apparatus.
German Offenlegungsschrift 39 13 885 discloses a boiler arrangement for burning moist fuel. This arrangement comprises a slanting grate, to the upper end of which the fuel is led. The fuel is dried on the upper part of the grate by conducting combustion gases for the purpose of drying. The fuel dries on the drying part of the grate and it is burnt on the remaining part of the grate. This arrangement has a big and expensive structure and it is not suitable for burning dry fuel.